Native Houdini Primitives

This addresses the first 3 bullets from #36 and is a start towards #37. Specifically, it provides the following:

 - Defines a new Houdini primitive for holding any IECore::Object
 - Provides basic viewport rendering using IECoreGL
 - Adds options to the SceneCache node for loading these new prims or regular Houdini geo
 - Allows them to be stored in hip, bgeo, and geo files
 - Allows these prims to be exported when caching scc files

These new primitives are not yet recognized by either Op/ProceduralHolders nor the To/FromHoudiniConverters. I plan to address those in a separate pull request, as it will require a significant re-jig of the API, and this bulk of work is already getting quite large.